An interesting read anytime but free through April 13th, 2014 (Just in time for Easter) this book by Dr. Wilson Jay Tyree delves into:
Early on Pentecost morning, the number of believers in that small band of outcasts barely broke the century mark. Luke, the author of the New Testament book of Acts, places the count at 120, mostly from the region around the Sea of Galilee. By the end of the day, their company would expand to include
Jewish pilgrims from over a dozen regions of the Empire and their numbers would exceed 3000. In this volume, we ask the question, “Why?”
Here there Be Dragons! 